Hybrid type filter networks



Nov. 17, 1959 D. KosowsKY 2,913,582

4 HYBRID TYPE FILTER NETWORKS Filed July 2, 1956 l l l y i i '2,913,682 y Y TrrE FILTER NETWORKS vDavid 1,. Kosowsky, West jNewton, `Mass., assignor to4 tion of Delaware y Appliaao July 2, 1956, serial No. 595,179 s claims. V(c1. ssa-72) kHermes 'Electronics Co., Cambridge, Ma,ss., a corporainvention relates to radio frequency wave Vfilters and, in particular, it relates to crystallters formed with hybrid ,coils in a pseudo-lattice configuration.

Because of its inherent generality, the symmetrical lattice is especially well suited for bandpass crystal filter applications, .and indeedmost bandpasslrters, incorporating-piezoelectric crystals as frequency selective elements,

have thisconfiguration or its equivalent. Such alltermay include two pairs of crystals Vper section, a iirst pair of identical crystalsY being arrange'clin the .respective series arms A of the lattice and a second pair of identical-,crystals being arranged in the respective shunt arms. The di- .culty with this arrangement -is thatV extraordinarily care is required to match the crystals sufficiently closely, and

.therefore it isfusually the practice to employ a divided Plate .QrvStaL ,that is, @Crystal wherein the Platina 91 .each E electrode `is divided into `two electrically insulated areas,

in place of each matched pair of crystals. This expedient, although satisfactory at lower frequencies, is impractical at relatively high frequencies, because of practical ditliculties in performing the electrode division, and then, too,it is relativelycostly. The equivalent of the lattice or pseudo-lattice formed with a hybrid coil avoids these is directed.

In the drawing: 1

Fig. 1 is a schematic diagram of a conventional single section pseudo-lattice or hybrid `type crystal filter network;

. Fig. -2is a schematic diagram of the latticeequivalent of the network of Fig. l in idealized form;

the networks of Figs. l and 2;

Fig. 4 is a schematic diagram lof a conventional twosection hybrid type crystal liilternetwork;

` Fig. 57is aschematic diagram of the lattice'equivalent of the network of Fig. 4 in videalized form;

Fig. 6 is a graphof the attenuation characteristic of the A,networks of Figs. `4 `and 5;

Fig. 7 is a schematic diagram of ,the lattice equivalent of the network of Fig. 4 in more complete form;

Fig. 8 is a schematic diagram of a two-section hybrid type crystal filter network according to the invention;

Figs. 9 and l0 lare simple variations of the network of .Fig 8 also iniaccordmance with ftheinvention;` and Fig. l1 is a schematic diagram of another two-section hybrid type crystal lter network also in accordance with the invention.

With referencewnow to the drawing, there is illustrated .in Fig. l.l a conventional hybrid typecryjstal filter network ,for narrow passband applications. As shown, .this vnetfwork comprises a three-winding hybridcoil H :apair ,of-,rystals-QA and 2B whose characteristics determine" the attenuation characteristics lof the network. The input of the network is constituted by a pair of terminals 1, 1 connected to the primary winding of the hybrid, and the output of the network is constituted by a pair of terminals 2,913,682 e Patented Nov. 17, 1959 f2, 2', the formerfbeing connected to the junction of the crystals 2A and 2B, and the latter being connected to the junction of the balanced secondary windings of the hybrid. tlf the hybrid. transformer is idealized, that is considered to have windings of infiniteV inductance and unity coupling (between the primary and secondary windings and beand if the total number of turns `orithe secondary Wind-V difficulties, andit is to an improvement ofrcrystall filter ,networks of this latter type that `the present invention Y Fig. 3 is a graph of the attenuation characteristic of tween theindividual secondary windings themselves),

ing is (twice that of the primary winding, itcan be shown Vthat thelattice network 'of Fig. 2nis inall respects equival- Vent to the network ofFig. l. 'As shown in Fig. 2, this network comprises a pair of identical crystals A in the respective series arms ofthe lattice and a pair of identical crystals B in therespective shunt arms. Crystals A and 'B are like the crystals 2A and 2B of the network of Fig.

-'l, respectively, except that their impedance is one half that of crystals 2A and 2B, as the notation indicates. .Even though an idealized form of hybrid transformer' is `unrealizable in.practice,both the networks of Figs'. l and 2 .exhibit substantially the same attenuation characteris tics. Fig. 3 illustrates the attentuation characteristic a of these networks as a functionof frequency f.

` In Fig. 4 there is illustrated. a crystal'lilter network comprising .essentially two ofthe Asections of Fig. l but .havingonly one hybrid H1 with a single pair of balanced windings. iThus, a first pair of crystals 2A1, 2B1 in series combination are connected across the ends of the hybrid windings along with the series. combinaiton of a second .pair of crystals 2A2, ZBZ. Atterminal 3 connected to the junction of crystals 2A1, 2B1 and a terminal 3 connected to the junction of the hybrid windings constitute the input circuit ofthe network. Theoutput circuit is constituted yby apairofrterminals 4, 4', the Vformer being connected .to the junction ofcrystals 2A2, ZBZ and lthe .latter being connectedto the same point asterminal 3. A capacitor Czis also .provided across the hybrid windings to resonate in the pass band with the coupled inductances L2 thereof so that these inductance valueswill not affect, appreciably, the characteristics of the network that are of interest.

it be assumed that the. hybrid H1' has a unity coupling coeicient K2. between windings, the lattice equivalent ofthe network of Fig. 4 vis as .shown in Fig. 5, crystals.

A1, "B1, A2 and B2 having one half .the impedance of their counterparts in the hybrid type network of Fig. 4. Although, as aforementioned, the attenuation characteristicsof the networks of Figs. 1 and 2 roughly correspond tto one another even though an ideal hybrid coil is not realizable in practice, the attenuation characteristic x4 of .the network of Fig. 4, it has been found, does not measure up to that of the network of Fig. 5. This is shown in Fig. 6 where the attenuation characteristic a., as a function of frequency f is .represented .by the dotted line, Iand the attenuation characteristic a5 of the network of Fig. 5 is represented by the solid line. With reference to Fig. 6, it will be observed that a marked decrease in the --attenuation @characteristic a., occurs .in `the stop iband adjacent the lower lend of the pass band. Because of its proximity to the pass band, this decrease or dip is particularly undesirable since it degrades the specification ofmaxirnum attenuation correspondingly when the network of Fig. 4 is employed instead of theV network of Fig. 5. The hybrid `type networkaccording to the invention, as illustrated in Fig. -8, avoids this disadvantage, permitting the f ull capabilities of the lattice configuration to be realized.

To show why this is so, it is necessary rst to analyze the network of Fig. 4 in more complete form, taking into account normal coupling values between windings of the hybrid coil H1 as determined by their respective leakageinductances. In Fig. 7 where such complete form of lattice equivalent of the network of Fig. 4 is L1=% 1-k2) and nea-ih faz) If the coefficient of coupling between the windings k2 ,were unity, it is evident that L1 and L3 would vanish, reducing the network to the idealized form of Fig. 5. In practice, however, k2 Yusually has a value from between 0.80 to 0.95 which establishes the value of L1 in the. range between .1L2 `and roughly .01L2. Such values of inductance L1 provide an inductive reactance in the region just below the pass band which is comparable to the reactance of the idealized form of network of Fig. 4 or its equivalent of Fig. 5. That is to say, when lthe network of Fig. 5, for example, is operated under normal conditions, such as image impedance conditions with resistive terminations at either end, the respective input .and output impedances of the network can be shown to have a reactive component substantially equal to the reactance ZwL1 produced in the input and output circuits due to the leakage inductances of the hybrid windings, where w equals 21rf. Since the former is capacitive in this region, a series resonance effect is apparently produced by the leakage inductances which accounts for the degradation in the attenuation a4. In the network of Fig. 8, the leakage inductances characterized by L1 and L3 are neutralized, permitting the attenuation characteristic a5 to be realized with a hybrid type network.

With reference once again to Fig. 8, it will be observed that the critical distinction between this network and the network of Fig. 4 is that a capacitor C is provided in each terminal line of the network of Fig. 8 to resonate with the apparent or virtual inductances L1 at the pass band center frequency fo or thereabouts. Thus, if

is equal to 21rfL1 when .f equals fo, the effect of the vinductances L1 will be eliminated. Since crystal filters are generally narrow band anyway, that is the band width is usually less than 0.8% of the center frequency, Athe same holds true as a practical matter about so much -of the pass band as is usually of interest. If then C2 is assigned a value which takes into account inductances L3 as well as coupled inductances KZLZ, it has been found that the network of Fig. 8 has the same characteristics as that of Fig. 5 including substantially the same attenuation a5.-

Figs. 9 and 10 illustrate variations of the network of Fig. 8. In Fig. 9 two capacitors are employed in series with the respective hybrid windings, capacitors being exactly half the size of capacitors C as the notation indicates. Fig. 11 illustrates still another variation wherein only a single capacitor C is provided between the junction of the hybrid windings and the common terminals 3' and 4. Those skilled in the art will recognize that despite the slightly different appearances of the networks of Figs. 8 through 10, the same are the exact equivalents of one another.

Like considerations are applicable to a wide band crystal filter except that more precise means must be provided to eliminate the effects of the hybrid winding leakage inductances, owing to the substantially larger frequency region of interest. Thus, as shown in Fig. 11, inductance and capacitance combinations of Lp and Cp are provided between the respective junctions of the vhybrid windings (two hybrids are required in this conventional form of wide band crystal filter) and the common terminals 3 and 4. If then the values of the inductance Lp and capacitance Cp are assigned such that their combined reactance characteristic as a function of frequency is equal and opposite to that of the inductance 2L1 over the frequency region of interest, it is evident that the same beneficial results will be obtained in the wide band filter case as in the narrow band filter case. Various such modifications as this to meet the peculiar requirements of other conventional hybrid type crystal filter networks wliich are nevertheless within the spirit and scope of the invention will no doubt occur to those skilled in the art, and therefore what is claimed is:

1. In a hybrid type, band selective, crystal filter network having a transformer with at least a pair of balanced windings, the combination with said transformer of a capacitive circuit effectively connected in series with said balanced windings to series resonate in the vicinity of the selection band with the virtual inductances produced by the leakage inductances of the balanced windings, thereby to enhance the attenuation characteristics of the lter network.

2. In a hybrid type, band selective, crystal filter network having a transformer with at least a pair of balanced windings, the combination with said transformer of a capacitor l effectively connected in series with each balanced winding and having a value which is related to the virtual inductances L1 produced by the leakage inductances of the balanced windings according to the formula ara where fo is the selection band center frequency of the filter network.

3. In a hybrid type, band selective, crystal filter network having input and output terminals and a transformer with at least a pair of interconnected balanced windings, the combination with said transformer of a capacitor C connected between the point of interconnection of the balanced windings and one of said output terminals and having a value which is related to the virtual inductances L1 produced by the leakage inductances of the balanced windings according to the formula l iVm-211.0131

where fo is the selection band center frequency of the filter network.

References Cited in the file of this patent UNITED STATES PATENTS 2,169,301 Sykes Aug. 15, 1939 2,266,658 Robinson Dec. 16, 1941 2,278,801 Rust et al. Apr. 7, 1942 2,333,148 Botsford Nov. 2, 1943 

